Lithography systems are commonly used in fabricating large-scale integrated circuits such as computer chips and the like. The function of a lithography system is to direct spatially patterned radiation onto a photoresist-coated wafer. The process involves determining which location of the wafer is to receive the radiation (alignment) and applying the radiation to the photoresist at that location (exposure).
In general, a lithography system, also referred to as a lithography tool, an exposure system or an exposure tool, typically includes an illumination system and a wafer positioning system. The illumination system includes a radiation source for providing radiation such as ultraviolet, visible, x-ray, electron, or ion radiation, and a reticle or mask for imparting the pattern to the radiation, thereby generating the spatially patterned radiation. In addition, for the case of reduction lithography, the illumination system can include a lens assembly for imaging the spatially patterned radiation onto the wafer. The imaged radiation exposes resist coated onto the wafer. The illumination system also includes a mask stage for supporting the mask and a positioning system for adjusting the position of the mask stage relative to the radiation directed through the mask. The wafer positioning system includes a wafer stage for supporting the wafer and wafer chuck and a positioning system for adjusting the position of the wafer stage relative to the imaged radiation. Fabrication of integrated circuits can include multiple exposing steps. For a general reference on lithography, see, for example, J. R. Sheats and B. W. Smith, in Microlithography: Science and Technology (Marcel Dekker, Inc., New York, 1998), the contents of which is incorporated herein by reference.
During exposure, the radiation source illuminates the patterned reticle, which scatters the radiation to produce the spatially patterned radiation. The reticle is also referred to as a mask, and these terms are used interchangeably below. In the case of reduction lithography, a reduction lens collects the scattered radiation and forms a reduced image of the reticle pattern. Alternatively, in the case of proximity printing, the scattered radiation propagates a small distance (typically on the order of microns) before contacting the wafer to produce a 1:1 image of the reticle pattern. The radiation initiates photo-chemical processes in the resist that convert the radiation pattern into a latent image within the resist.
Interferometry metrology systems herein after referred to simply as interferometer systems are typically important components of the positioning mechanisms that control the positions of the wafer and reticle and register the reticle image on the wafer. Interferometry systems can be used to precisely measure the positions of each of the wafer stage and mask stage relative to other components of the exposure system, such as the lens assembly, radiation source or support structure. In such cases, the interferometry system can be attached to a stationary structure and a measurement object attached to a movable element such as one of the mask and wafer stages. Alternatively, the situation can be reversed, with the interferometry system attached to a movable object and the measurement object attached to a stationary object.
More generally, such interferometry systems can be used to measure the position of any one component of the exposure system relative to any other component of the exposure system, in which the interferometry system is attached to, or supported by, one of the components and the measurement object is attached, or is supported by the other of the components.
There are various reasons that favor the operation of a lithography tool with the cavity of the lithography tool filled with a gas instead of where the cavity is evacuated. However, the presence of a dispersive medium such as the gas in the measurement and reference paths of an interferometric system used to monitor the position of the stage or stages of the lithography tool introduces uncertainty in measurements made using the interferometric system due to the atmospheric effects.
A difficult measurement related to the refractive index of a gas is the compensation of refractive index fluctuations over a measurement path of unknown or variable length, with uncontrolled temperature and pressure. An example situation is high-precision distance measuring interferometry, such as is employed in micro-lithographic fabrication of integrated circuits. See for example an article entitled “Residual Errors In Laser Interferometry From Air Turbulence And Nonlinearity,” by N. Bobroff, Appl. Opt. 26(13), pp 2676-2682 (1987), and an article entitled “Recent Advances In Displacement Measuring Interferometry,” also by N. Bobroff, Measurement Science & Tech. 4(9), pp 907-926 (1993). As noted in the aforementioned cited references, interferometric displacement measurements in a gas are subject to environmental uncertainties, particularly to changes in air pressure and temperature; to uncertainties in air composition such as resulting from changes in humidity; and to the effects of turbulence in the gas. Such factors alter the wavelength of the light used to measure the displacement. Under normal conditions the refractive index of air for example is approximately 1.0003 with a variation of the order of 1×10−5 to 1×10−4. In many applications the refractive index of air must be known with a relative precision of less than 0.1 ppm (parts per million) to less than 0.001 ppm, these two relative precisions corresponding to a displacement measurement accuracy of 100 nm and less than 1 nm, respectively, for a one meter interferometric displacement measurement.
There are frequent references in the art to heterodyne methods of phase estimation, in which the phase varies with time in a controlled way. For example, in a known form of prior-art heterodyne distance-measuring interferometer, the source emits two orthogonally polarized beams having slightly different optical frequencies (e.g. 2 MHz). The interferometric receiver in this case is typically comprised of a linear polarizer and a photodetector to measure a time-varying interference signal. The signal oscillates at the beat frequency and the phase of the signal corresponds to the relative phase difference. A further representative example of the prior art in heterodyne distance-measuring interferometry is taught in commonly owned U.S. Pat. No. 4,688,940 issued to G. E. Sommargren and M. Schaham (1987). These known forms of interferometric metrology do not generally compensate for fluctuations in refractive index of a gas in a measurement path of an interferometer.
One way to detect refractive index fluctuations is to measure changes in pressure and temperature along a measurement path and calculate the effect on the optical path length of the measurement path. Mathematical equations for effecting this calculation are disclosed in an article entitled “The Refractivity Of Air,” by F. E. Jones, J. Res. NBS 86(1), pp 27-32 (1981). An implementation of the technique is described in an article entitled “High-Accuracy Displacement Interferometry In Air,” by W. T. Estler, Appl. Opt. 24(6), pp 808-815 (1985). This technique provides approximate values, is cumbersome, and corrects for slow, global fluctuations in air density.
Another, more direct way to detect the effects of a fluctuating refractive index over a measurement path is by multiple-wavelength distance measurement. The basic principle may be understood as follows. Interferometers and laser radar measure the optical path length between a reference and an object, most often in open air. The optical path length is the integrated product of the refractive index and the physical path traversed by a measurement beam. In that the refractive index varies with wavelength, but the physical path is independent of wavelength, it is generally possible to determine the physical path length from the optical path length, particularly the contributions of fluctuations in refractive index, provided that the instrument employs at least two wavelengths. The variation of refractive index with wavelength is known in the art as dispersion and this technique is often referred to as the dispersion technique.
An example of a two-wavelength distance measurement system is described in an article by Y. Zhu, H. Matsumoto, T. O'ishi, SPIE 1319, Optics in Complex Systems, pp 538-539 (1990), entitled “Long-Arm Two-Color Interferometer For Measuring The Change Of Air Refractive Index.” The system of Zhu et al. employs a 1064 nm wavelength YAG laser and a 632 nm HeNe laser together with quadrature phase detection. Substantially the same instrument is described in Japanese in an earlier article by Zhu et al. entitled “Measurement Of Atmospheric Phase And Intensity Turbulence For Long-Path Distance Interferometer,” Proc. 3rd Meeting On Lightwave Sensing Technology, Appl. Phys. Soc. of Japan, 39 (1989). The interferometer of Zhu et al. has insufficient resolution for applications that require sub-micron interferometry such as microlithography.
An example of a two-wavelength high-precision interferometry system for microlithography is represented by U.S. Pat. No. 4,948,254 issued to A. Ishida (1990). A similar device is described by Ishida in an article entitled “Two Wavelength Displacement-Measuring Interferometer Using Second-Harmonic Light To Eliminate Air-Turbulence-Induced Errors,” Jpn. J. Appl. Phys. 28(3), L473-475 (1989). In the article, a displacement-measuring interferometer is disclosed which eliminates errors caused by fluctuations in the refractive index by means of two-wavelength dispersion detection. An Ar+ laser source provides both wavelengths simultaneously by means of a frequency-doubling crystal known in the art as BBO. The use of a BBO doubling crystal results in two wavelengths that are fundamentally phase locked, thus greatly improving the stability and accuracy of the refractive index measurement. However, the motion of the object results in rapid variations in phase that make it difficult to detect accurately the effects of fluctuations in the refractive index.
In U.S. Pat. No. 5,404,222 entitled “Interferometric Measuring System With Air Turbulence Compensation,” issued to S. A. Lis (1995), there is disclosed a two-wavelength interferometer employing the dispersion technique for detecting and compensating refractive index fluctuations. A similar device is described by Lis in an article entitled “An Air Turbulence Compensated Interferometer For IC Manufacturing,” SPIE 2440 (1995). Improvement on U.S. Pat. No. 5,404,222 by S. A. Lis is disclosed in U.S. Pat. No. 5,537,209. The principal innovation of this system with respect to that taught by Ishida in Jpn. J. Appl. Phys. (supra) is the addition of a second BBO doubling crystal to improve the precision of the phase detection means. The additional BBO crystal makes it possible to optically interfere two beams having wavelengths that are exactly a factor of two different. The resultant interference has a phase that is directly dependent on the refractive index but is substantially independent of stage motion.
Two two-wavelength distance measuring systems based on superheterodyne techniques are described in commonly owned U.S. Pat. No. 5,764,362 entitled “Superheterodyne Method And Apparatus For Measuring The Refractive Index Of Air Using Multiple-Pass Interferometry” by Henry A. Hill and P. de Groot and U.S. Pat. No. 5,838,485 entitled “Superheterodyne Interferometer And Methods For Compensating The Refractive Index Of Air Using Electronic Frequency Multiplication” by Peter de Groot and Henry A. Hill. The contents of both of the two cited patents are herein incorporated in their entirety by reference. In both of the two referenced patents, contributions to measured phases due to effects of a gas in a measurement path are directly dependent on the refractive index but the contributions due to stage motion are substantially reduced. The first of the two referenced patents is based on multiple pass interferometry and the second referenced patent is based on electronic frequency multiplication.
Other commonly owned U.S. Patents relating to dispersion interferometry are U.S. Pat. No. 6,330,065 B1, U.S. Pat. No. 6,327,039 B1, U.S. Pat. No. 6,407,866, U.S. Pat. No. 6,525,825, U.S. Pat. No. 6,525,826 B2, U.S. Pat. No. 6,529,279 and U.S. Pat. No. 6,219,144 B1. The contents of the other commonly owned cited patents are herein incorporated in their entirety by reference.
A commonly owned U.S. Patent relating to the measurement of intrinsic properties of a gas such as the reciprocal dispersive power is U.S. Pat. No. 6,124,931 (Γ monitor). The contents of the commonly owned cited patent are herein incorporated in their entirety by reference.
A non-dispersive apparatus and method for the compensation of turbulent effects of a gas is described in commonly owned U.S. patent application Ser. No. 10/350,522 entitled “Method and Apparatus For Compensation Of Time-varying Optical Properties of Gas In Interferometry” by Henry A. Hill. Patent application Ser. No. 10/350,522 compensates for turbulent effects of the gas on the direction of propagation of a first beam by using measured effects of the gas turbulence on the directions of propagation of the first beam and a second beam. The first and second beams are spatially separated and the directions of propagation of the first and second beams are substantially parallel. Gas turbulence effects on the measurement path length of the first beam are compensated by using the measured turbulent effects of changes in the direction of propagation of the first beam over the measurement path length and a known relationship between the effects of gas turbulence on the direction of propagation of a beam and on the corresponding effects on the optical path length. The contents of cited patent application Ser. No. 10/350,522 are incorporated herein in their entirety by reference.
Another non-dispersive apparatus and method for the compensation of turbulent effects of a gas is described in commonly owned U.S. patent application Ser. No. 10/701,759 entitled “Compensation of Refractivity Perturbations In A Measurement Path Of An Interferometer” by Henry A. Hill. Patent application Ser. No. 10/701,759 compensates for turbulent effects of the gas on the optical path length of a beam or the average optical path length of two beams of an interferometer system by using measured differential effects of the gas turbulence at a single wavelength on the relative measurement path lengths of a first beam and a second beam wherein cells of the gas that pass through the measurement path of the first beam are subsequently transported through the measurement path of the second beam. The directions of propagation of the spatially separated first and second beams are substantially parallel. The contents of cited U.S. patent application Ser. No. 10/701,759 are incorporated herein in their entirety by reference.
Another non-dispersive apparatus and method for the compensation for turbulence effects of a gas is described in commonly owned U.S. Provisional Application No. 60/676,190 entitled “Compensation of Turbulent Effects Of Gas In Measurement Paths of Multi-Axis Interferometers,” the corresponding utility application Ser. No. 11/413,917, and the CIP of U.S. patent application Ser. No. 11/413,917 also entitled “Compensation of Turbulent Effects Of Gas In Measurement Paths of Multi-Axis Interferometers.” The provisional and the two utility applications are by Henry A. Hill and the contents of the cited provisional and the two utility applications are incorporated herein in their entirety by reference.
The effects of stationary changes in the optical path length of measurement paths at a single wavelength are compensated by using a procedure such as described in commonly owned U.S. Pat. No. 7,075,619 B2 entitled “In-Process Correction of Stage Mirror Deformations During a Photolithography Exposure Cycle” and U.S. Pat. No. 6,842,256 entitled “Compensation For Effects Of Variations In Gas Refractivity In Interferometers.” The two U.S. patents are by Henry A. Hill and the contents of both thereof are incorporated herein in their entirety by reference.